Tono Flashcards
.Forces involved in Goldmann tono
Obtain equilibrium at one pt
1) Applanation force + 2) Capillary attraction of tears = 3) Corneal elasticity [resistance to bending] + 4) Intraocuar pressure [IOP]
.Applanation - Definition
Abnormal flattening of cornea [e.g. using tonometer tip]
.Applanation diameter
3.06mm = Sig b/c when 1 g of applanation F = 10 mmHg of IOP
Within range where
- capillary attraction = corneal elasticity
- applanation F = IOP
.Tono mire variations - Not centered L to R
No error induced
.Mean corneal thickness
0.536 mm +/- 0.031 mm
Thinner (flattens easily) -> Underestimate
Thicker (flattens difficultly) -> Overestimate
10% diff in corneal thickness results in 1.1-2.5 mmHg diff in IOP
.IOP overestimation - Causes
1) Increased corneal thickness
2) Excessive tears, fluorescein
3) Lid retraction
4) Lash contact
5) Not having half circle mires on either side of line
6) Tight shirt collar
7) Valsalva maneuver / Hold breath
8) Acute acc
.Appearance of mires if pulsing
High IOP - Mires spread apart
Diameter too small - Req increased F
Low IOP - Mires overlap
Diameter too large - Req decreased F
.Tono tip adjustment for astigmatism
If > 3D of astig, based on change in Ks
Set red line to flatter meridian
If not adjusted, reading would be overestimated
.Normal IOPs
Avg = 15 - 16 mmHg +/- 5 Range = 10 - 21 mmHg
Rarely diff by more than 3 mmHg btwn eyes
Diurnal variation 3-6 mmHg (commonly > in morning)
.IOP underestimation - Causes
1) Decreased corneal thickness
2) Insufficient tears
3) Lack of fluorescein
4) Poor illumination
5) Corneal edema
6) Multiple applanations
7) Sustained acc
.Imbert-Fick Law
When flat surface pressed against spherical surface, equilibrium attained when F balance
Assumptions about sphere
- Infinitely thin
- Perfectly elastic
- Perfectly flexible
- No other F
- Rel large surface area/ volume
